Alerting configured for error rate spikes; thresholds defined; alert destinations configured

An error rate that spikes from 0.1% to 15% without triggering an alert means your team finds out from a user tweet, not a PagerDuty notification. NIST SI-4 and NIST CSF DE.AE-3 require automated detection of anomalous system events; ISO 25010 reliability.fault-tolerance requires that faults be detected and responded to. Without defined thresholds and alert destinations, error rate monitoring is passive — a dashboard that nobody checks during an incident rather than an active signal that wakes someone up.

Configure an error rate alert in your monitoring service. For Datadog:

1. Monitors → New Monitor → Metric
2. Query: avg:trace.web.request.error_rate{service:your-app} by {env}
3. Alert threshold: > 0.05 (5%) for 5 consecutive minutes
4. Notification: @slack-incidents @pagerduty-oncall

For Prometheus + Alertmanager:

# prometheus-alerts.yml
groups:
  - name: app
    rules:
      - alert: HighErrorRate
        expr: rate(http_requests_total{status=~"5.."}[5m]) / rate(http_requests_total[5m]) > 0.05
        for: 5m
        annotations:
          summary: "Error rate {{ $value | humanizePercentage }} on {{ $labels.instance }}"

Document the alert owner, threshold justification, and expected response steps in INCIDENT_RESPONSE.md.